One type of microwave magnetic device that has long been recognized for its performance capabilities is the ferrite phase shifter. For example, ferrite phase shifters are known for their low insertion loss, microwave power-handling capabilities, high reliability, and high radiation tolerance. In conventional ferrite phase shifter devices, operation is typically based on electromagnetic wave propagation in low-loss magnetic materials, such as yttrium iron garnet (YIG) and spinel ferrites with cations of lithium, magnesium, nickel, and zinc. Further, such devices are typically biased using permanent magnets and/or current-driven coils.
However, conventional ferrite phase shifter devices have several drawbacks. For example, when such devices are biased using permanent magnets, the size, weight, and cost of the devices can increase, especially for ferrite phase shifters that are designed to operate at high frequencies (e.g., at or above X-band). Further, such devices biased using permanent magnets provide virtually no significant tunability of their operating frequencies. A degree of tunability can be achieved when the ferrite phase shifter devices are biased using current-driven coils. However, like the devices that are biased using permanent magnets, the devices biased using current-driven coils can also have increased size, weight, and cost. Further, such devices biased using current-driven coils generally exhibit increased DC power consumption, and long response times (e.g., on the order of milliseconds) due to the relatively large inductance of the coils. The use of such conventional ferrite phase shifter devices has therefore generally been limited to applications in which low insertion loss and high power handling capability prevail over essentially all other design considerations.
In order to reduce the DC power consumption and to improve the response times, some conventional ferrite phase shifter devices have incorporated latching-type ferrite phase shifters, which employ short current pulses to set the phase of the devices. For example, such latching-type ferrite phase shifters are typically designed using waveguide components and toroidal-shaped ferrite cores. Further, the toroidal shape of the ferrite cores provides a flux closure path, which can reduce the current drive requirements, and increase remnant magnetization within the ferrite cores. However, such devices that incorporate latching-type ferrite phase shifters can also have increased size and weight due to the relatively large waveguide components. The cost of fabricating such devices with waveguide components can also be high. Moreover, because the short current pulses used to set the phase of the devices typically provide discrete phase settings, such devices incorporating latching-type ferrite phase shifters have generally been incapable of providing the high level of accuracy required for critical applications, such as phased array radar systems.
In the conventional ferrite phase shifter devices described above, the operating frequencies have traditionally been tuned, when possible, using magnetic field tuning techniques, e.g., by changing the magnetic fields applied to the respective devices. However, such magnetic field tuning of ferrite phase shifter devices can be slow, and can require a considerable amount of power. To avoid the drawbacks of magnetic field tuning, electric field tuning techniques have been employed in some conventional ferrite phase shifter devices. In such devices, a ferrite layer is typically bonded to a piezoelectric layer to form a ferrite/piezoelectric composite element. Further, to tune the operating frequencies of such devices, an electric field is created in the composite element to produce, via the magnetoelectric (ME) effect, a mechanical strain in the piezoelectric layer that transmits a force to the ferrite layer. The force transmitted to the ferrite layer of the composite element manifests itself as an internal magnetic field that can change the phase shift of the electromagnetic waves propagating through the ferrite layer.
However, electric field tuning of conventional ferrite phase shifter devices also has several drawbacks. For example, for optimum results, the ferrite layer of the ferrite/piezoelectric composite element included in such devices should have a large magnetostriction constant. However, ferrite materials that have low insertion loss generally exhibit low magnetostriction. For this reason, such devices have typically been designed to operate near the ferromagnetic resonance (FMR) frequency, where small internal magnetic fields generated via the ME effect in the ferrite layer can cause large variations in the magnetic permeability of the ferrite layer, thereby facilitating tuning of the devices. However, operating such conventional ferrite phase shifter devices near FMR can limit the bandwidth of the devices, and cause increased electromagnetic wave propagation losses.
It would therefore be desirable to have tunable microwave magnetic devices, and methods of tuning such microwave magnetic devices, that avoid at least some of the drawbacks associated with the conventional microwave magnetic devices described above.